My DD has the same issue... Hers was considereded 'moderate to severe' but it's only the back of her head. Hers was caused because when she was born, her clavacle broke. So she couldn't get ANY tummy-time or side-sleeping time for the first 4 weeks. She didn't even like to turn her head, so even if we were holding her, it was the back of her head taking the brunt of the weight. She is almost a year old now, and in the last couple of months I have noticed a lot of change. Her head has started rounding out on it's own! I think it helps that I 'massage' her scalp, and kind of 'pet' it putting a little more pressure on the wide, and no pressure on the flat spots. Her head isn't 'normal' yet, but I have noticed the difference. So have other people, based on the pictures they have seen of her. Even her pediatrician says that it's not nearly as bad as it was. I have also really pushed her to sleep on her side/stomach too, and keep the pressure off the flat spot.
Generally, you can fix her head with the helmet up until she turns 2 years (MAXIMUM) old. (that's when the head stops growing so fast) After that, it usually requires surgery. You could always do like we did, and give it a 'chance' to round out with the option of using the helmet later. The main downside to that though is that if she DOES need the helmet, she will have to wear it longer.
Our decision was made because of our finances. We just couldn't afford the expensive helmet, and all the office visits. When we recieved our tax return, we stashed away most of it so IF we decide she does need the helmet (if she stops improving, or isn't improved enough as we get closer to that 2 year mark...) we can afford to get it for her. If not, she has a decent start toward her college fund. :)
Good luck, whichever way you decide to go! :)